概述

A {@link Deque} that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.

Deque支持阻塞操作：当获取元素时会等待Deque变成非空；当存储元素时会等待Deque变可用；

{@code BlockingDeque} methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future:
one throws an exception,
the second returns a special value (either {@code null} or {@code false}, depending on the operation),
the third blocks the current thread indefinitely until the operation can succeed,
and the fourth blocks for only a given maximum time limit before giving up.

BlockingDeque方法有4种形式：

1、抛异常

2、依赖不同的操作，返回null或false；

3、阻塞当前线程直到操作成功

4、阻塞直到timeout放弃

Like any {@link BlockingQueue}, a {@code BlockingDeque} is thread safe, does not permit null elements, and may (or may not) be capacity-constrained.

BlockingDeque是线程安全的，不允许null，可能也有容量限制；

一个双端队列；

在不能够插入元素时，它将阻塞住试图插入元素的线程；在不能够抽取元素时，它将阻塞住试图抽取的线程。

deque(双端队列) 是 "Double Ended Queue" 的缩写。

因此，双端队列是一个你可以从任意一端插入或者抽取元素的队列；